1. Field of the Invention
This invention relates generally to improvements in air compressors, and more particularly to a method and apparatus for intermediate stage dehydration in a multistage air compressor using a membrane type dehydrator. This invention is more reliable and environmentally friendly than conventional compressor/dryer systems which use mechanical chillers, moisture separators, drain systems, and desiccant type dehydrators because it requires no moving parts, no electrical heating supply, no desiccants which cause downstream contamination, and no CFC's. Because of these features it also requires less maintenance.
2. Description of the Prior Art
The amount of moisture contained in free air is dependent on the temperature and pressure of the air. As air is compressed, the moisture holding capacity is reduced when the volume is reduced if the temperature is held constant. However, as air is compressed, the temperature increases, thereby increasing the moisture-holding capacity. This increased air temperature is in itself generally undesirable for most compressed air applications. Also, the temperature of areas downstream may be lower than that of the moisture-laden compressed air, and condensation results which is generally undesirable. In most instances, heat exchangers (aftercoolers) are used to lower the temperature of the air after each stage of compression. By lowering the temperature, some moisture in the form of liquid condensate is removed from the air. The liquid contaminants in most compressed air systems are water and oil, or an emulsified combination of the two. These are removed by mechanical means such as moisture separators or coalescing filters. However, compressed air is still laden with water vapor which is not easily removed by mechanical means but is best removed by the process known as air drying.
In the prior art the three commonly used methods of air drying are (1) absorbent (deliquescent desiccant), (2) adsorbent (regenerative desiccant), and (3) refrigeration (mechanical refrigerated cooling). Each of these methods has advantages and disadvantages. Deliquescent dryers have no moving parts and low initial cost. However, deliquescent dryers have limited dew point suppression; 20.degree. to 30.degree. F. is common. They also have high maintenance, requiring periodic replacement of the desiccant, and they must be manually drained regularly. Dryers using regenerative desiccants on the other hand are able to achieve low dew points, to -100.degree. F., but have high initial cost, high operating cost, and require periodic servicing of desiccant towers. Refrigeration dryers have low maintenance and low operating cost, but cannot produce low dew points. Dew points are limited to 38.degree. F. as a minimum to prevent freezeup. Refrigeration-type dryers are commonly used as a first step dryer ahead of desiccant type dryers.
A fourth type of dryer has been developed based on the use of semipermeable membranes. Semipermeable membranes have long been employed in gas separation operations. Gases pass through the membrane by a combination of diffusion through the pores linking the surfaces of the membrane and permeation through the material of the membrane. The driving force for the separation process is the difference between the partial pressure of a gas on one side of the membrane, and the partial pressure of a gas on the other side of the membrane. When gaseous mixtures are compressed, the partial pressures of the various gas components increase. In the case of compressed air, the partial pressure of the water vapor will be equal to the saturation pressure at the corresponding temperature of the gaseous mixture. When the compressed air is cooled, the partial pressure of the water vapor in the duty air stream could be lower than the surrounding air. Therefore, the driving force for separation of water vapor from compressed air is achieved by lowering the partial pressure of the water vapor on the outside of the membranes. This is done by encasing the membranes in a shell which functions as a conduit to transport the permeated water vapor and as a barrier to separate the membranes from the wet atmospheric air. A dry carrier gas is produced which is used to purge the atmospheric air from the inside of the shell and as "sweep air" to carry water vapor away from the membrane. The sweep air and water vapor are continuously vented from the shell of the dryer.
New developments in membrane fabrication and packaging techniques have made membrane technology attractive for large scale high pressure air drying. A semipermeable membrane has been developed which has a design pressure limit of 1000 psig. We have discovered a way to incorporate this new membrane in a high pressure air compressor system.